Various semiconductor devices such as microcomputers, memories, and gate arrays are provided in many electric products, for example, personal computers and workstations. In most cases, semiconductor devices include input/output pins for external data transmission and an output circuit for outputting data. For example, the output circuit has output buffers and drivers.
When semiconductor devices are provided in electric products, their input/output pins are connected to corresponding transmission lines such as wires on a board. Internal data from a semiconductor device is provided to another semiconductor device through transmission lines that form an interface. In order for output data of a semiconductor device to be properly transmitted through transmission lines, impedance between input/output lines and transmission lines should be matched.
As operation speeds of electric products increase, a swing width of a signal interface between semiconductor devices decreases. This allows a delay time to be reduced at signal transmission. However, as the swing width of a transmitted signal decreases, the semiconductor device may be affected by external noise and reflection of an output signal due to an impedance mismatch at the interface. Impedance mismatch is caused by external noise or by power supply voltage, temperature and process variations. If an impedance mismatch exists, it may be difficult to transmit data at a high speed, and data from a semiconductor device may be distorted. Thus, in a case where a semiconductor device receives distorted data, problems can be caused by a setup/hold fail or erroneous judgment of received data.
Conventional semiconductor memory devices have adopted impedance correcting techniques for matching impedance with other devices. For example, in an HSTL (high speed transceiver logic) interface, a technique is used for controlling impedance so as to have a desired output impedance value within several tens ohms using one extra pin. In such a semiconductor device, however, it is difficult to obtain an output impedance equal to a designed value due to power supply voltage, temperature, and process variations.
To overcome the aforementioned problems, generally, a trimming circuit having a metal, bonding or fuse operation is provided in a semiconductor device. Impedance matching is accomplished by adjusting a reference voltage for impedance matching and a resistance value range using the trimming circuit. This technique is called a programmable impedance control function. However, this technique may require considerable test time and/or an additional process for adjusting impedance. The above problems are limiting factors in decreasing fabrication cost.